This invention relates generally to thin-film sol-gel coatings and, more particularly, to curing thin-film sol-gel coatings applied to substrates having a low melting temperature.
Sol-gel materials have found numerous uses in commercial and industrial products, including for example forming near net shape objects, encasing optical fibers, and providing antireflection coatings for display devices. Sol-gel coatings typically are formulated by mixing together an alkoxide, an alcohol, and water to produce a pre-polymerized solution, or sol. The pre-polymerized solution is applied to a substrate by any of several methods, including dip coating, spin coating, spray coating, gravure coating, and meniscus coating. Each such application causes a prescribed amount of the solution to adhere to the substrate. The adhered solution is then cured to form a separate polymerized layer on the substrate. In many applications, particularly in the case of optical coatings, multiple sol-gel layers comprising different sol compositions with different optical indices can be applied to the substrate, in order to achieve desired optical properties. U.S. Pat. No. 5,856,018, issued to Chen, et al., which is incorporated by reference, describes one suitable use of sol-gel coatings for producing an antireflection coating.
In all cases, it is necessary to properly cure the wet sol layer after it has adhered to the substrate. Curing, which usually is accomplished by applying heat energy in an oven, evaporates residual organics and other liquid compounds of the solution from the adhered layer. The curing process, performed at elevated temperatures for a certain time duration, densifies the layers. Generally, the higher the temperature, the better the cure; and the longer the exposure to temperature, the better the cure. A trade-off exists between the duration of time the coating is held at an elevated temperature and the value of that temperature. Higher temperatures require a shorter exposure time. The temperature preferably is selected to be the maximum temperature that the particular substrate can withstand without deformation. The temperature, as well as the duration of the cure, affects the mechanical strength of the resulting layer, such as its scratch resistance or its adhesion. An incomplete cure will result in reduced mechanical properties.
Difficulties can arise when the substrate is formed of a low melting point material such as polymethyl methacrylate (PMMA), polycarbonate (PC), or other plastics. In such cases, the cure temperature must be maintained below about 100 to 150° C., depending on the particular substrate material, to avoid melting or warping the substrate. To provide sufficient curing energy at these low temperatures for achieving satisfactory densification and mechanical strength, long curing times, on the order of tens of minutes or even hours, typically are required. This can increase substantially the processing time and cost of the product, sometimes making the product economically non-viable.
It should, therefore, be apparent that there is a need for an apparatus and method for rapidly curing sol-gel coatings applied to low-melting point substrates, without warping or otherwise damaging the substrates, which yields dense and mechanically strong coatings, with a relatively short processing time. The present invention fulfills this need.